Growth cones, first described by Cajal in 1890, are dilations at the tip of growing axons. They are intriguing in that these structures apparently direct the growth of the axon, mediate the recognition of and then engineer the contacting of the appropriate cell by the axon. Growth cones could thus be considered the chemosensors of the growing axon. The ultrastructure of growth cones in rats has been studied (del Cerro, 1968 and 1974), but relatively little has been done in the study of invertebrate growth cones (see Meinertzhagen 1973). The complexity of the developing mammalian brain severely restricts the study of changes evoked within cells touched by a growth cone and also the ultrastructural changes occuring within the growth cone itself. Meinertzhagen (1973) has stated that basic understanding of the behavior of growth cones in any one region is certain to have widespread significance in the analysis of neuropile development. It is the intent of this project to pursue an understanding of this growth cone behavior in Drosophila, which in turn may clarify the basic mechanisms in the formation of the complex visual connections of higher organisms and even man. The numerous visual system mutants in Drosophila should, by comparative study, aid in elucidating the normal structure and function. The Drosophila also fits midway in size and complexity between the simplest and most complex forms (Benzer, 1971). Relevant data will be obtained by relatively routine electron microscopic procedures. Thick sections of plastic embedded tissue will be used in optical microscopy for general orientation. Thin sections will be prepared to show ultrastructural detail of the growth cones. The overall objective of the project is to use the ultrastructural detail as a basis of interpretation of function in the retinular growth cones and their associated filopods.